<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8"/>
        <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
        <meta http-equiv="X-UA-Compatible" content="ie=edge"/>
        <title>Document</title>
        <style>
            div{width: 100px; height: 100px; background-color: red; margin: 50px; opacity: 0.3; filter: alpha(opacity=30);}
        </style>
        <script>
            window.onload = function(){
                var aDivs = document.getElementsByTagName("div");
                for(var i = 0; i < aDivs.length; i++){
                    aDivs[i].alpha = 30;
                    aDivs[i].onmouseover = function(){
                        startMove(this, 100);
                    }
                    aDivs[i].onmouseout = function(){
                        startMove(this, 30);
                    }
                }
            }

           
            /* 
                中间变量:多个物体在进行透明度变化的时候，公用的是一个中间变量。

                结论：任何的变量在多物体运动中，都不能共用。
            */
            // var alpha = 30;

            function startMove(node, iTarget){
               clearInterval(node.timer);
               node.timer = setInterval(function(){
                   var speed = (iTarget - node.alpha) / 8;
                   speed = speed > 0 ? Math.ceil(speed) : Math.floor(speed);

                   if(node.alpha == iTarget){
                       clearInterval(node.timer);
                   }else{
                       node.alpha += speed;

                       node.style.opacity = node.alpha / 100;
                       node.style.filter = "alpha(opacity=" + node.alpha + ")";
                   }

               }, 30);
            }
        </script>
    </head>
    <body>
        <div></div>
        <div></div>
        <div></div>
        <div></div>
    </body>
</html>